Sensory gating, the brain's ability to modulate its sensitivity to sensory input, is an important neural protective and regulatory function. Deficits of this function have been found in a number of psychiatric conditions and may contribute particularly to the development of psychotic symptoms. These deficits can be studied by recording the P50, a positive cerebral evoked potential (EP) component peaking about 50 milliseconds following auditory stimulation. However, the cerebral structures mediating sensory gating and generating the P50 are not well known. Therefore, the main goal of the current proposal is to identify cerebral structures contributing to sensory gating and generating the P50 EP component by recording intracranial EPs directly from the human cerebral cortex and the hippocampus. Approximately 160 patients with medically intractable epilepsy in whom the presurgical evaluation necessitates the implantation of intracranial electrodes will be included in the study. EPs will be recorded from intrahippocampal depth and subdural strip electrodes in different cerebral regions. These data will be accumulated to map sensory gating processes onto the human cerebral cortex and the limbic system. In particular, the data will allow the examination of our hypothesis that the temporo-parietal region plays an important part in mediating sensory gating. In addition, intracranial sensory gating indices will be related to subject's psychiatric symptomatology (based on widely used rating scales). These data will shed more light on this crucial function and will help further our understanding of the mechanisms that lead to the development of psychotic disorders.